Observation of Metallization Transition of 2D Alkali Metal Films

The deexcitation rate of metastable ${}_{}{}^3{}_{}{}^{}S$ ${\mathrm{He}}^{*}$ atoms was measured during the growth of Na, K, and Cs films on a Cu(100) surface. As a consequence of the reduced work function, which is caused by adsorption of small amounts of alkali metals, the ${\mathrm{He}}^{*}$ deexcitation mainly takes place via Auger deexcitation. Since the incident ${\mathrm{He}}^{*}$ atoms interact only with the vacuum side tails of the electronic states of the surface atoms, the deexcitation rate for this mechanism reflects the electronic surface density of states. At coverages of about 0.4 monolayer a dramatic increase in the deexcitation rate was obtained, which is attributed to the subsequent filling of the alkali metal valence band and thus the onset of metallization.